1. Field of the Invention
The present invention relates to an antenna circuit using a magnetic sensor and a watch provided with the antenna circuit.
2. Description of Related Art
In recent years, a technique to form an antenna with a magnetic sensor using a magneto-impedance element (MI element) has been proposed (for example, see JP 2000-188558A). As disclosed in JP 2000-188558A, when minute radio frequency current is applied to a soft magnetic material formed in wire shape, ribbon shape or the like, there occurs output voltage across the soft magnetic material due to the impedance thereof. The magneto-impedance effect represents an effect that impedance of soft magnetic material changes sensitively when external magnetic field is applied thereto so that output voltage across the soft magnetic material changes.
FIG. 18 is one example of an antenna circuit to detect magnetism by a magnetic sensor using a MI element (hereinafter referred to as “MI magnetic sensor”) and a filter circuit following after the antenna circuit. In the Figure, S1, R1, C1 and Z1 represent a radio-frequency signal generator, a resistance, a condenser and a MI magnetic sensor, respectively. In the magnetism detection circuit of FIG. 18, a radio-frequency signal from the radio-frequency signal generator S1 is distributed to the resistance R1 and MI magnetic sensor Z1, and is output through the condenser C1. When AC magnetic field shown as a dashed line is applied to the MI magnetic sensor Z1, the signal of the radio-frequency signal generator S1 is distributed according to the correlation between the magnetic field and impedance of the MI magnetic sensor shown in FIG. 19A, so that the signal shows magneto-impedance change according to the AC magnetic field. Since the magnetic field changes between plus and minus and the output is based on an even function having an axisymmetric sin curve, the frequency component of the output becomes twice as much as the original frequency.
When fixed magnetic field (see reference numeral 1901) as shown in FIG. 19B is further applied, change of external magnetic field is output with being centered on this fixed magnetic field. Since the curve is approximately linear around the fixed magnetic field in FIG. 19B, the resistance changes in proportion to external magnetic field. That is, since the inversion due to the above-described axisymmetric curve does not occur, the magnetic resistance change have the same frequency.
In the antenna circuit of FIG. 18, the signal at point a is represented as A sin ωt according to S1, and the voltage at point a is represented as Va=A sin ωtZ1/(R1+Z1) according to the magnetic sensor exposed to magnetic field change.
Since the MI magnetic sensor Z1 has variable impedance according to magnetic field change, Z1 can be represented as Z1=Z(1+B sin pt). Here, since B<<1, the equation of Va can be transformed as follows.
                                                        Va              =                            ⁢                              A                ⁢                                                                  ⁢                sin                ⁢                                                                  ⁢                ω                ⁢                                                                  ⁢                                  t                  ·                  Z                                ⁢                                                                  ⁢                1                ⁢                                  (                                                            R                      ⁢                                                                                          ⁢                      1                                        +                                          Z                      ⁢                                                                                          ⁢                      1                                                        )                                                                                                        =                            ⁢                              A                ⁢                                                                  ⁢                sin                ⁢                                                                  ⁢                ω                ⁢                                                                  ⁢                                  t                  ·                                                            Z                      (                                              1                        +                                                  B                          ⁢                                                                                                          ⁢                          sin                          ⁢                                                                                                          ⁢                          pt                                                                    )                                        /                                          (                                                                        R                          ⁢                                                                                                          ⁢                          1                                                +                                                  Z                          ⁡                                                      (                                                          1                              +                                                              B                                ⁢                                                                                                                                  ⁢                                sin                                ⁢                                                                                                                                  ⁢                                p                                ⁢                                                                                                                                  ⁢                                t                                                                                      )                                                                                              )                                                                                                                                              ≈                            ⁢                              A                ⁢                                                                  ⁢                sin                ⁢                                                                  ⁢                ω                ⁢                                                                  ⁢                                  t                  ·                                                            Z                      ⁡                                              (                                                  1                          +                                                      B                            ⁢                                                                                                                  ⁢                            sin                            ⁢                                                                                                                  ⁢                            p                            ⁢                                                                                                                  ⁢                            t                                                                          )                                                              /                                          (                                                                        R                          ⁢                                                                                                          ⁢                          1                                                ⁢                                                                                                  +                        Z                                            )                                                                                                                              (        1        )            
Formula (1) has the same format as that of amplitude modulation (AM), which shows that the signal of the radio-frequency signal generator S1 is subject to amplitude modulation according to magnetic field frequency. On the other hand, the signal of the radio-frequency signal generator S1 is a radio-frequency signal for generating skin effect of the magnetic sensor. The impedance change in the magnetic sensor modulates the signal of the radio-frequency signal generator S1. As shown in FIG. 20A, the signal of the radio-frequency signal generator S1 is modulated to be a state represented by reference numeral 2002 according to magnetic field change (see reference numeral 2000). Thus, a waveform as shown in FIG. 10A is propagated from point a or the condenser C1. By employing a configuration to receive this waveform with a circuit equivalent to a AM receiver, it becomes possible to receive magnetic field change. Here, since the modulated signal has been subject to amplitude modulation, there occurs side bands 2011 and 2012 with respect to a carrier signal 2010 as shown in FIG. 20B. Since these side bands changes according to magnetic field change, it is preferable to detect them. However, as shown in formula (1), the value of B is very small (several percent), the side bands are extremely small with respect to the carrier.
Here, receiver sensitivity is to be considered. When it is assumed a general wave clock has receiver sensitivity of 40 dBμ/m for example, the sensitivity can be converted to 10−8Oe (1Oe≈79.6 A/m). According to a magnetic permeability of vacuum, it can be regarded that 1Oe≈1G. Accordingly, it is required that an antenna applied to a wave clock has sensitivity of 10−8G.
However, a MI sensor of earlier development having a normal sensor shape has sensitivity of about 50 mV/G. Even if the sensor can detect 1 μV signal, detection of magnetic field becomes 1/(5×104) thereof. That is, although 10−8 G receiving sensitivity is required, only reception at 2×10−5 G sensitivity is possible. Thus, a MI sensor of earlier development is lacking in sensitivity for a wave clock.
In order to improve sensitivity, the following means could be given.
(1) To devise shape of a MI sensor itself to reduce effect of demagnetizing field.
(2) To improve detection accuracy of side bands
An object of the present invention is to improve detection accuracy of side bands, so as to provide a receiving apparatus which can detect a desired signal from received electric wave and a watch provided with the receiving apparatus.
For example, it is assumed that a received signal (40 KHz) has C/N of 140 dB with respect to a 20 MHz signal of the radio-frequency signal generator S1, and that the signal of S1 is applied to the resistance R1 at 3 Vrms. It is preferable for reducing power consumption that impedance of S1 side at a point (point a) between the resistance R1 and MI magnetic sensor Z1 is large. Accordingly, when it is assumed that R1=Z1=1 MΩ, the impedance of S1 side becomes 500 kΩ. In this case, thermal noise value Vn is given as follows when it is assumed that bandwidth B is 10 Hz and absolute temperature T is 300.
                    Vn        =                  20          ⁢                                          ⁢          log          ⁢                      √                          (                              4                ⁢                kBTR                            )                                                              =                                            -              77.8                        ⁢                                                  ⁢            dB            ⁢                                                  ⁢            µ                    +                      10            ⁢                          log              ⁡                              (                BR                )                                                                            =                              -            77.8                    +          67.0                                        =                              -            10.8                    ⁢                                          ⁢          dB          ⁢                                          ⁢          µ                                        =                  0.29          ⁢                                          ⁢          µVrms                    
The total noise value is 3.01 μVrms, and applied signal noise is predominant among them.
In order to reduce the above thermal noise, application of signal correlation is known. Thus, synchronous detection may be employed using a phase comparator 81, a low-pass filter 82, a oscillator 83 and a mixer 84 as shown in FIG. 21, instead of the envelope detector circuit provided with diodes D1 and D2 as shown in FIG. 18. However, in the synchronous detection of earlier development as shown in FIG. 21, it is problematic that synchronization using the phase comparator 81 and oscillator 83 is difficult due to noise.
Further, a magnetic sensor disclosed in JP 2000-188588A detects magnetic field change in wide frequency range because of lacking filter property. Thus, it is impossible to detect only magnetic field change at specific frequency.
Such configuration requires a tuning circuit to select desired frequency component provided at a later stage of an antenna section. Moreover, since such receiving apparatus uses an MI sensor, it requires a radio-frequency AC power source (driving circuit) to drive a magneto-impedance element. As a result, size of the antenna section becomes comparatively large.
It is an object of the present invention to provide an antenna circuit in which detection accuracy of side bands is improved by reducing noise from a radio-frequency signal generator with a comparatively simple circuit, and a watch provided with the antenna circuit.
It is also an object of the present invention to downsize an antenna apparatus, a receiving apparatus and electronic equipment, without decreasing the receiving sensitivity thereof.